Diamond is a preferred material for microelectronic devices because it has semiconductor properties that are superior to conventional semiconductor materials, such as silicon, germanium or gallium arsenide. Diamond provides a higher energy bandgap, a higher breakdown voltage, and a higher saturation velocity than these traditional semiconductor materials.
These properties of diamond yield a substantial increase in projected cutoff frequency and maximum operating voltage compared to devices fabricated using more conventional semiconductor materials. For example, silicon is typically not used at temperatures higher than about 200.degree. C. and gallium arsenide is not typically used above 300.degree. C. These temperature limitations are caused, in part, because of the relatively low energy band gaps for silicon (1.12 eV at ambient temperature) and gallium arsenide (1.42 eV at ambient temperature). Diamond, in contrast, has a relatively high band gap of 5.47 eV at ambient temperature, and is thermally stable up to about 1400.degree. C. in a vacuum.
Diamond also has the highest thermal conductivity of any solid at room temperature and exhibits good thermal conductivity over a wide temperature range. The high thermal conductivity of diamond may be advantageously used to remove waste heat from an integrated circuit, particularly as integration densities increase. In addition, diamond has a smaller neutron cross-section which reduces its degradation in radioactive environments, that is, diamond is a "radiation-hard" material. Diamond is also relatively chemically inert, optically transparent, and mechanically hard. Accordingly, diamond can be used advantageously in optical applications, and its mechanical hardness means that it is robust and can be used as an extremely effective abrasive agent. These mechanical properties also produce excellent acoustic characteristics.
Because of the advantages of diamond as a material for microelectronic devices, there is at present an interest in the growth and use of diamond for devices which can be used in environments which are subjected to high temperatures, radiation, and/or corrosive agents. For example, there is an interest in the use of diamond for sensors, thermal management devices, and electron beam devices such as field emitters and electron-activated switches. There is also an interest in the use of diamond for Surface Acoustic Wave ("SAW") devices because of the relatively high velocity of surface acoustic waves through diamond. SAW devices including diamond layers are discussed in U.S. Pat. Nos. 5,329,208, 5,355,568, and 4,952,832, all to Imai et al.
Unfortunately, the fabrication of a single crystal diamond film is typically carried out by homoepitaxial deposition of a diamond film on a single crystal diamond substrate. Such a single crystal diamond substrate is relatively expensive. In addition, large single crystal substrates may not be available for many applications.
A continuous layer of diamond, however, may not be suited for the large scale production of diamond devices or structures because a wafer with a continuous diamond layer may be difficult to cut into individual die. The ability to efficiently cut the production wafer into individual die is important because economies of scale dictate that many devices be fabricated simultaneously on a single wafer and then cut apart after fabrication. While substrates made from conventional materials such as silicon can be cut using a mechanical saw, a substrate including a diamond layer may require a more complicated cutting tool such as a laser because of the extreme hardness of diamond. Lasers, however, may be relatively expensive, and may induce micro cracks or other damage in the diamond. The use of lasers may also cause adhesion problems as a result of localized heating and thermal expansion, formation of non-diamond phases along the edges of cuts, and ablation of carbon residue onto devices.
A proposed microelectronic device having one or more semiconductor devices formed on a single crystal substrate, such as diamond, is described in U.S. Pat. No. 5,006,914 entitled "Single Crystal Diamond Substrate Articles and Semiconducting Device Comprising Same" to Beetz, Jr. et al. This patent discloses a microelectronic structure including a single crystal diamond substrate which is etched to form an array of spaced apart posts of single crystal diamond. On each post is grown a semiconducting layer of single crystal diamond to serve as an active channel region of a respective semiconductor device. Unfortunately, the use of a large single crystal diamond substrate as the starting point for the fabrication of the Beetz structure is relatively expensive. In addition, the diamond substrate may be difficult to cut into individual die.
Another microelectronic structure is described in U.S. Pat. No. 5,420,443 entitled "Microelectronic Structure Having An Array Of Diamond Structures On A Nondiamond Substrate And Associated Fabrication Methods" to Dreifus et al. The '443 patent is assigned to the assignee of the present invention, and it represents a significant advance in the state of the art. The '443 patent and the present invention also share common inventors.
The microelectronic structure of the '443 patent includes a single crystal non-diamond substrate, and a plurality of laterally spaced apart diamond structures are formed on the substrate extending outwardly therefrom. An interfacial carbide layer is preferably formed between the plurality of diamond structures and the non-diamond substrate, and the diamond structures are substantially oriented with respect to the non-diamond substrate. The diamond structures preferably have a substantially flat outermost face having a (100)-orientation to thereby provide a relatively large usable area in contrast to other crystalline orientations. The embodiment of the method of this patent provides nucleation of an array of diamond structures, each approaching single crystal quality without scratching or abrading the surface of the substrate.
Still another microelectronic structure is described in U.S. Pat. No. 5,300,188 entitled "Process For Making Substantially Smooth Diamond" to Tessmer et al. The '188 patent is also assigned to the assignee of the present invention, and it also represents a significant advance in the state of the art. The '188 patent and the present invention also share common inventors.
The '188 patent discusses a process for making a diamond layer having a substantially smooth upper surface and a predetermined thickness on a substrate. The process includes depositing a patterned polish stopping layer on a substrate to a predetermined thickness while leaving predetermined portions of the substrate exposed. In particular, the polish stopping layer is preferably a layer of a material such as various metals, polysilicon, silicon nitride, silicon oxide or other suitable materials capable of substantially stopping the consumption of diamond. A diamond layer is then deposited on the polish stopping layer and on the predetermined portions of the substrate left exposed.
Notwithstanding the above mentioned references, there continues to exist a need in the art for improved methods for forming diamond structures which can be used in the fabrication of microelectronic devices. There also exists a need in the art for economical methods for forming diamond structures which can be subsequently processed using conventional microfabrication techniques.